1. Field
The invention herein disclosed relates to optical tape and methods and systems of production and use thereof.
2. Description of the Related Art
Vacuum roll coating has long been used to deposit single and multiple layers of metallic and non-metallic materials on flexible substrates. One particular advantage of vacuum roll coating may be in its ability to coat large substrate areas, with the largest vacuum coating machines being capable of handling rolls of substrate exceeding 10 feet in width and coating speeds in excess of several thousand feet per minute (41st Annual Technical Conference Proceedings, Society of Vacuum Coaters, Boston Mass. 18 Apr., 1998, pg. 26).
Vacuum roll coaters utilize one or more techniques to deposit the desired film layers, the most common techniques being thermal evaporation, electron beam (e-beam) evaporation, and sputtering. The first two are characterized by relatively high material deposition rates but generally do not produce as high a degree of deposition uniformity as the sputtering process.
Deposition quality in roll coating may be often stated in terms of transverse (cross-web) and longitudinal (machine direction) uniformity, corresponding to thickness or compositional variations across the width and along the length of the roll, respectively. Cross-web and machine direction variations can arise from several sources, including non-uniform spatial and temporal distributions of the flux from the material source.
There are a number of techniques known to the art to control and minimize such fluctuations, including use of sensors to control the rate of material deposition through a feedback means (crystal monitors, reflectometers, etc.), which typically affect temporal variations from the source, and shutters or e-beam scanning, which typically affects the spatial material distribution.
It may be typically more difficult to maintain an extremely high degree of uniformity in the cross-web direction than the machine direction, particularly with high deposition rate e-beam and thermal evaporation processes. Contributing factors include non-uniform heating, depletion of the source material during the deposition process, material buildup at the source, etc. Uniformity can decrease as the material may be depleted until the coating process must stop to refill the material reservoirs, although in larger systems the material may be replenished continuously, for example by means of wire or screw fed devices. For precision coatings requiring very high thickness tolerances, the cross-web uniformity achievable by high deposition rate thermal and e-beam processes may be often unacceptable.
Following the vacuum coating process, it may be common to slit the coated material into narrower widths, such as in the case of tape-like materials. The slitting process typically uses a mechanical means to effect the slitting, such as a knife box or other shearing device, and this operation can be a source of problems for the coated substrate. Slitting can result in disruption or delamination of the coating at the slit edges, with the concomitant generation of coating and substrate particles. The particulates thus generated not only contaminate the slit rolls, but they also can be incorporated into the spools under tension during rewind, which can irreversibly damage the rolls, a particular problem in the case of sensitive coatings or substrates.
It may also be common in tape-like substrates to coat both the front and back surfaces. This requires either a second pass through the coating machine or incorporation of a tandem coating station and related web-handling equipment, both of which can add to the cost of process and/or equipment.
The vacuum coating method described herein has been developed to substantially eliminate the shortcomings and disadvantages of the prior art as noted above, and thereby provide a means for coating tape-like substrates with improved uniformity, edge quality, cleanliness, and at higher throughput.